The output voltage and frequency of permanent magnet (PM) alternators are essentially determined by the rotor speed, which present challenges where the rotor speed cannot be independently controlled, such as in a gas turbine engine. Also, controllability in low- or no-load conditions may require using a regulation apparatus, such as a shunt regulation system, to draw current from the machine if the voltage increases above a given maximum allowable value. But these systems add weight and complexity. Instead, in one solution presented in U.S. Pat. No. 7,262,539, a set of control windings is provided around a portion of the stator.
The number of control winding turns generally sets the desired inductance of the control windings. However, the number of turns and the inductance can be the subject of conflicting requirements for designers. For example, a lower number of turns can be desirable to minimize voltage loss and reduce heating. However, increasing the number of turns to increase the inductance may be desirable to minimize current ripple or for other benefits. To date, the designers must sometimes make hard choices in optimizing inductance and machine performance, especially where the machine is used as both a generator and a motor. Room for improvement thus exists.